Chore: Refactor Prometheus HTTP client middleware (#34473)

Following #33439 this refactors the Prometheus HTTP transport 
which is replaced by HTTP client middleware.

pkg/models/datasource_cache.go

@@ -50,7 +50,7 @@ func (ds *DataSource) GetHTTPClient(provider httpclient.Provider) (*http.Client,
 	}, nil
 }
 
-func (ds *DataSource) GetHTTPTransport(provider httpclient.Provider) (http.RoundTripper, error) {
+func (ds *DataSource) GetHTTPTransport(provider httpclient.Provider, customMiddlewares ...sdkhttpclient.Middleware) (http.RoundTripper, error) {
 	ptc.Lock()
 	defer ptc.Unlock()
 
@@ -58,7 +58,10 @@ func (ds *DataSource) GetHTTPTransport(provider httpclient.Provider) (http.Round
 		return t.roundTripper, nil
 	}
 
-	rt, err := provider.GetTransport(ds.HTTPClientOptions())
+	opts := ds.HTTPClientOptions()
+	opts.Middlewares = customMiddlewares
+
+	rt, err := provider.GetTransport(opts)
 	if err != nil {
 		return nil, err
 	}